Electronic devices, such as calculators, watches, portable computer terminals, and "smart" cards, utilize a wide variety of integrated circuits, rigid circuits, and flexible circuits. In such devices, there are a number of means used for attaching and providing electrical connection between a circuit and a conductive substrate. The means for providing attachment and electrical connection include metallic solders and adhesive compositions, including conductive adhesive compositions.
Conductive adhesives may contain sparse and randomly dispersed populations of fine conductive particles. The particles provide electrical conductivity through the thickness of the adhesive, but not in the plane of the adhesive film. Such adhesives are sometimes provided as an adhesive film, often referred to as Z-axis films (ZAF).
Adhesives that are used in known Z-axis adhesive films typically have glass transition temperatures (T.sub.g) of less than about 50 .degree. C., and are usually soft, rubbery, thermoplastic materials having little or no crosslinking. The use of such an adhesive permits rapid bonding, at a modest temperature, with ease of repairability.
A repairable, or reworkable, adhesive is an adhesive that can be entirely removed from the bonded device with heat and/or solvent. The use of a reworkable adhesive allows for the bonded device to be removed and repositioned without damage to the device, e.g., a printed circuit or glass display. Displays or boards containing more than one chip are expensive. If one chip is defective, it is highly desirable to be able to replace it.
A deficiency in soft, rubbery, thermoplastic adhesives used for electrical connection is that they typically have poor creep resistance at high stress and/or temperature. Accordingly, such adhesives are useful only in applications involving limited stress. Thus, such adhesives are almost exclusively used for the bonding of lightweight, flexible circuits to other components, an application with relatively minimal stress. An example of such a thermoplastic adhesive for use as a repairable adhesive film in electronic applications is disclosed in U.S. Pat. No. 4,820,446 (Prud'Homme).
While certain highly crosslinked or thermoset adhesive systems perform adequately in high stress applications, these adhesives are deficient in that they are not reworkable. An example of such a non-reworkable adhesive composition, exhibiting superior shear strengths at high temperatures, is disclosed in U.S. Pat. No. 4,769,399 (Schenz).
An application in the electronic industry with particularly demanding performance requirements has arisen, at least in part, from the demand for increased power to size ratio in electronic products, and the resultant need for higher input/output (I/O) counts and I/O densities between components. In addition, the demand for higher information flow rates has necessitated higher signal frequencies. To accommodate these needs, Flipped Direct Chip Attachment (FDCA) is used to attach a chip directly to a circuit board, and provide the shortest attainable path length between components, thus minimizing signal propagation delays at high frequency.
The most common attachment means used in FDCA is solder bump/flip-chip interconnection. With this approach, metallurgical solder joints provide both the mechanical and electrical interconnections between the chip and the substrate. This method has inherent pitch limitations and also is susceptible to mismatches between the coefficients of thermal expansion (CTE) of the chip and the substrate. Substantial CTE mismatch between these materials results in high shear stresses in the solder joints which can compromise the reliability of the interconnections. See, R. R. Tummala and E. J. Rymaszewski, Microelectronics Packaging Handbook, (Van Nostrand Reinhold, 1989) pp. 280-309; 366-391; and K. Nakamura, Nikkei Microdevices, June 1987. Catastrophic failure is the immediate result of any cracking that occurs in these solder joints as a reaction to these stresses.
Because of the inherent deficiencies with solder bump bonding, an alternate means of interconnection for FDCA and other applications is highly desirable. One method involves the use of a heat-bondable adhesive, which may or may not be curable, to provide an intimate mechanical flip-chip bond, and to provide pressure engaged, rather than metallurgical, electrical interconnections to the substrate. Conductive particles within the adhesive or, alternately, metallic bumps on the chip itself, provide the electrical interconnection media for this method. Thermal and/or cure shrinkage stresses in the adhesive can be used to provide the compressive forces on the interconnection media that are needed in order to establish the pressure engaged contacts. In order to permit the buildup of contacting forces on the interconnection media, it is necessary that the interconnection media remain in the solid state during the bonding operation.
Pressure engaged interconnections are more able to accommodate shear stresses than are solder joints. Also, the presence of an adhesive at the bond line will tend to dramatically reduce the magnitude of the shear stresses at the contact sites. Therefore, CTE mismatches are better accommodated by FDCA with adhesives than by solder bump bonding.
Another potential advantage of FDCA with adhesives arises from the fact that the interconnection media (i.e., conductive particles in the adhesive or metallic bumps on the chip) do not melt during the bonding process, as does, for example, solder. It is much easier to control pad-to-pad spacing when the conductive structures in the bond line remain in the solid state during processing. Because of this, FDCA with adhesives provides the potential for reduced, and also more precise, spacing between interconnections as compared to solder bump bonding.
An important obstacle preventing the use of adhesives for FDCA has been the lack of an adhesive that has adequate creep resistance at elevated temperature and stress to provide stable interconnections and that also provides ease of handling and repairability. After bonding, the adhesive layer is highly stressed in tension. In order to provide a stable interconnection, the adhesive layer must be able to sustain this internal stress without creeping or fracturing. The high stress is the result of: a) the adhesive layer being as much as twenty times thinner than either a silicon chip or a rigid printed circuit board; and b) the very high CTE and very low elastic modulus of adhesives relative to either of these substrates.
The magnitude of the tensile stress in the adhesive layer is sufficiently high that known thermoplastic resins that are heat processable at temperatures of less than 200.degree. C. are unable to provide stable interconnections over the required range of operating temperatures. Highly crosslinked thermoset resins can be found that will provide stable interconnections at these high stresses and temperatures, but those materials are not reworkable with non-destructive methods.
In addition to the thermoplastic and thermoset adhesive systems, thermoplastic/thermoset blends are of possible interest for electrical interconnections. Such mixtures have been designed to improve high temperature performance of the thermoset materials, and/or to improve the fracture toughness of the thermoset material. See, U.S. Pat. No. 3,530,087 (Hayes et al.); and R. S. Bauer, Toughened High Performance Epoxy Resins: Modifications with Thermoplastics, 34th International SAMPE Symposium, May 8-11, 1989.
However, despite the prior uses of thermoplastic adhesives, thermoset adhesives, and mixtures thereof, presently known interconnect means for demanding applications, such as FDCA, have failed to adequately solve the problems set forth above. Accordingly, there remains a need for a reworkable adhesive material that permits rapid bonding at a modest temperature and that also has a modulus sufficient to withstand significant stress and/or high temperatures, at least through the range of use temperatures. The adhesive should also be reworkable at a processing temperature, T.sub.p, that is sufficiently low so that the substrate is not damaged during removal of a chip. There is also a need for such an adhesive that has an extended shelf life at room temperature, has a low viscosity at the intended bonding temperature to provide good flow properties, is resistant to conditions of up to 85.degree. C. and 85 percent relative humidity, is resistant to higher qualifying temperatures, and may be provided as a film that is substantially tackfree at the intended handling temperature.